The present invention relates to an optical fiber coupler which can be used in optical communication systems and optical sensors.
The optical fiber communication system makes use of an optical fiber coupler as an important element or part for separating and coupling, or branching and multiplexing light. In particular, an optical fiber coupler produced by processing optical fibers has widely been used in various fields, because of its excellent ability of being coupled with an optical fiber and a low propagation loss.
As will be seen from FIG. 1, an optical fiber coupler 10 comprises a plurality (two fibers are shown in this figure) of single mode optical fibers 1 and 2 and a light-coupling portion 11 formed by partially fusing, under drawing conditions, these optical fibers 1 and 2. Light rays incident upon one of these plurality of optical fibers are thus branched into the other optical fibers in a predetermined branching ratio.
FIG. 2 is a diagram showing main part Of a method for producing the optical fiber coupler 10. Protective coating materials 3 are previously removed from the desired portions of these optical fibers 1 and 2 to be fused together and the optical fibers are fixed by clamps 4 and 5 while the exposed portions of the fibers are brought into close contact with one another. The clamps 4 and 5 are connected to a tension mechanism which comprises a threaded bar 22, a guide bar 23 and a driving motor M. These two optical fibers 1 and 2 are fused by heating the contact portion thereof with a flame 6. If the driving motor M is put in operation at this stage, the clamps 4 and 5 each moves towards each corresponding direction indicated by an arrow and accordingly, the optical fibers 1 and 2 are drawn along the axial direction to thus give a tapered light-coupling portion 11. In this respect, the foregoing drawing operation is carried out while monitoring the branching ratio by making light rays emitted from a light source 7 having a desired wave length incident upon the optical fiber 1, detecting the intensity of the light outputted from the fiber 1 by a photodetector 8 and simultaneously detecting the intensity of the light outputted from the fiber 2 by a photodetector 9. When the branching ratio reaches a predetermined level, the driving motor M is interrupted to terminate the drawing operation.
The optical fiber coupler produced by the method discussed above has a very fine tapered light-coupling portion 11 and hence very low mechanical strength. For this reason, the fiber coupler causes axial deflection by the influence of even a weak external force which in turn results in a change of the branching ratio thereof. As a result, the optical fiber coupler exhibits unstable performance characteristics. Thus, the optical fiber coupler is adhered to a fixing stand in order to protect the coupler from the application of any external stress.
The optical fiber coupler of this type should have characteristic properties independent of environments in which the coupler is subsequently used. Japanese Patent Provisional Publication No. 3-107111 discloses a technique for stabilizing the characteristic properties of an optical fiber coupler of this type through the limitation of the Young's modulus of a resin used for the adhesion between the fiber coupler and a fixing container to a specific range. Moreover, Japanese Patent Application Publication No. 4-29046 discloses a technique which comprises supporting an optical fiber coupler on a buffering member and fixing the buffering member to a package at a point through the use of an adhesive.
Parts (including optical fiber couplers) generally used in optical fiber communication system are exposed to severe environmental conditions over a long time period and, therefore, it has been required for these parts to previously perform strict inspection thereof for the quality thereof. In respect of, for instance, tests on temperature change, the variation in properties before and after repeated heating/cooling cycle is limited to a specific range, while the variation in properties before and after application of a thermal shock is also limited to a specific range. Referring now to optical fiber couplers, those exhibiting performance characteristics stable to such a temperature change discussed above can be produced in only a low yield even when the foregoing techniques are adopted.